1. Field of the Invention
The present invention relates to a fluid-sealed vibro-isolating device which is suitable for an engine mount for automotive vehicles, and particularly to a fluid-sealed vibro-isolating device which is capable of effectively reducing or attentuating undesired vibrations, ranging in several kinds of frequency ranges.
2. Description of the Prior Art
As is generally known in the design of engine mountings, an internal combustion engine produces various kinds of vibrations or different vibration frequencies, ranging within a wide frequency range. When vibrating at low frequencies, ranging from 5 to 20 Hz (particularly close to 10 Hz), so-called "engine shake" may be excited by oscillatory hopping motion of wheels. During engine idling, the vibration frequency for the idle speed would be 20-'Hz or 25-50 Hz. When the engine frequency reaches a comparatively high frequency range of 80-200 Hz or 50-300 Hz, a high intensity vibration often called "booming noise" may be perceived audibly and also the acceleration period vibrations or noises may occur. As may be appreciated from the above, the so-called "engine shake" corresponding to low or intermediate frequency (5-20 Hz) vibrations involves relatively large amplitudes, whereas the so-called body-boom noise corresponding to high intensity vibration (50-200 Hz) involves relatively small amplitudes. The conflict between the high damping required for "engine shake" isolation and the low damping for dealing with noise has led to fluid-sealed vibration isolating mounts. One such fluid-sealed vibration isolating mount has been disclosed in Japanese Patent Provisional Publication No. 5-280576, assigned to the assignee of the present invention. FIG. 23 shows a simplified model of a vibrating system equivalent to the fluid-sealed vibration isolating device disclosed in the Japanese Patent Provisional Publication No. 5-280576. Hereinafter discussed is the detailed structure of the prior art vibration isolating device by reference to the simplified vibration model shown in FIG. 23.
Referring now to FIG. 23, in the conventional fluid-sealed vibro-isolating device, a primary damping-fluid chamber 1 of an expansion-phase elasticity K.sub.1 is communicated with a secondary damping-fluid chamber 2 of an expansion-phase elasticity K.sub.2 through two different fixed orifices, namely a first orifice 3 of a large equivalent mass and a second orifice 4 of a small equivalent mass. Also provided near the opening of the second orifice 4 is a fluid-flow restriction plate 5 for restricting the fluid flow from the primary chamber 1 to the secondary chamber 2 via the second orifice 4 when vibrating with an amplitude greater than a predetermined threshold value. In FIG. 23, K denotes an elasticity or a spring stiffness of the elastomeric rubber unit incorporated in the device. When large amplitude input vibrations such as "engine shake", that is, low or intermediate vibration frequencies (5-20 Hz) are transmitted from a vibrating body (engine) to the vibration isolating device (the vibrating system), the fluid flow passing through the second orifice 4 is effectively restricted by means of the flow restriction plate 5, since the amplitude of the input vibrations exceeds the predetermined threshold. As a result, only the first orifice 3 permits damping fluid flow between the primary and secondary chambers 1 and 2, while properly restricting the fluid flow therethrough. In the case that small-amplitude input vibrations such as idling vibration of the engine, that is, intermediate or high vibration frequencies (20-40 Hz or 25-50 Hz) are transmitted from the vibrating body (engine) to the vibration isolating device, the fluid communication between the primary and secondary chambers 1 and 2 is established by means of the two orifices 3 and 4, since there is no fluid-flow restriction of the restriction plate 5 owing to the amplitude of the input vibration less than the predetermined threshold. Therefore, in case of application of large-amplitude input vibration of low or intermediate frequencies, for example "engine shaking", such large-amplitude input vibration can be effectively reduced by way of liquid-column resonance of damping fluid within the first orifice 3 of the large equivalent mass. Alternatively, in case of application of small-amplitude input vibration of intermediate or high frequencies, for example idling vibration, such small-amplitude input vibration can be effectively reduced by way of total liquid-column resonance of damping fluid within both the first and second orifices 3 and 4. It is well known to set a loss factor at a great value for effectively damping low- or intermediate-frequency, large-amplitude vibrations, such as "engine shake", and to set a dynamic spring stiffness of the vibration isolating device (the vibro-isolating mount) at a small dynamic spring stiffness for effectively damping intermediate- or high-frequency, small-amplitude vibrations, such as "idling vibration of the engine". The Japanese Patent Provisional Publication No. 5-280576, teaches the use of the fluid-flow restriction plate 5 which is responsive to the amplitude of input vibrations for properly varying a combined equivalent mass of the first and second orifices 3 and 4, so that a great loss factor can be achieved in the presence of low- or intermediate frequency, large-amplitude vibrations (engine shaking, 5-20 Hz) and so that a small dynamic spring stiffness can be achieved in the presence of intermediate- or high-frequency, small-amplitude vibrations (idling vibrations, 20-40 Hz). In the prior art vibration isolating device discussed above, the size of the opening of the second orifice 4 can be adjusted to the minimum (a fully closed state) by means of the restriction plate 5 only when the large-amplitude input vibrations are transmitted to the device. Thus, it is possible to effectively reduce both low- or intermediate-frequency, large-amplitude vibrations (e.g., engine shake, approximately 10 Hz) and low- or intermediate-frequency, small-amplitude vibrations (e.g., idling vibration, 20-40 Hz). However, in the prior art device, there is a tendency for a dynamic spring stiffness (or a dynamic spring constant) to rapidly increase when the frequency of input vibration becomes higher than 40 Hz. The undesiredly increased dynamic spring stiffness of the device results in the lowering of vibration isolating performance with respect to input vibrations included in a high-intensity vibration range (boom noise) or a high-frequency vibration range (acceleration period noise), such as 50-300 Hz.
During driving on paved roads, the engine would experience a different kind of input vibration such as a low- or intermediate-frequency, small-amplitude vibration. The use of the flow-restriction plate 5 is effective to prevent or permit the flow of damping fluid passing through the second orifice 4 and consequently to damp engine shaking of a large amplitude or to damp the vibration caused by engine idling. However, the prior art device set out above cannot satisfactorily reduce the low- or intermediate-frequency, small-amplitude vibrations caused by paved-road running.